1. Field of the Invention
This invention relates to an amorphous thin magnetic film suitable for use as a soft adjacent layer in a magnetoresistive head, and in particular to a thin film of thickness from 200-1000 angstroms having long term stabilized uniaxial anisotropy.
2. Description Relative to the Prior Art
The use of thin magnetic films is well known in magnetic head technology. Such films are particularly useful in the fabrication of magnetoresistive (MR) elements or soft adjacent biasing layers (SAL) in magnetoresistive heads. The magnetic characteristics necessary in SAL applications, such as high saturation magnetization, high permeability, and low coercivity dictate that the thin films be amorphous; the crystalline phase generally exhibiting too high a coercivity and too low a resistivity for magnetic head use.
Among the materials suitable for SAL applications are permalloy, and amorphous alloys made from a transition metal such as cobalt and metal elements required for stabilizing the amorphous phase including Zr, Nb, Mo, Ta, Ru, Rh, Pd, Hf, Ti, W, Re, Os, and Ir. These materials lend themselves to sputtering techniques in the fabrication of amorphous thin films.
CoX alloys, where X is one of the elements listed above, are of particular interest as SAL elements because of their comparatively high magnetization saturation, high resistivity and low magnetoresistance. In a SAL type magnetoresistive head, a sense current flows through the MR element to provide the dual function of detection of the signal due to the change in the MR element resistance induced by the signal field, and to induce a magnetic field in the SAL which gives rise to the bias field at the MR element. When the soft adjacent layer is as conductive as the MR layer, a troublesome problem has been the shorting of the soft adjacent layer element to the magnetoresistive element due to, for example, pin holes in very thin insulation separating the two elements. Under this condition, a relatively conductive SAL material, such as permalloy having a conductivity of approximately 20 ohm-cm), will shunt the sense current in the MR element resulting in head performance deterioration. Additionally, if the SAL has an appreciable magnetoresistance characteristic it will itself act as an MR element and contribute to the overall magnetoresistive changes of the head due to the signal field, with additional detriment to performance. The CoX alloys have proven an effective substitute for permalloy as a soft adjacent layer material. ("CoZrMo Amorphous Films as a Soft Adjacent Layer for Biasing Magnetoresistive Elements with a Current Shunt Layer", Yamada et al, 32nd Annual Conference on Magnetism and Magnetic Materials, Chicago, Ill., November, 1987.) Because of its higher resistivity in the range of 100 micro-ohm-cm a CoX soft adjacent layer, when used with a permalloy MR element, will only shunt about 1/5 of the sense current from the magnetoresistive element if shorts occur between the magnetoresistive element and the soft adjacent layer. Since the magnetoresistance for amorphous films is smaller by a factor of 10 of that of permalloy, even if current flows in the SAL due to a short, its contribution to the magnetoresistive change of the head is negligible.
While the CoX alloys show promise in alleviating problems arising in thin film head construction, the CoX alloys known in the prior art do not exhibit stable magnetic anisotropy field values when fabricated in the optimum thickness for an SAL. In a magnetoresistive head having soft adjacent layer biasing, it is desirable that the soft adjacent layer be operated in a saturated state making the bias field at the magnetoresistive element independent of changes in the sense current which generates the bias field. This, in turn, means the SAL must be thin enough so that the field of the sense current saturates it. In general, the optimum SAL thickness is in the range 200-1000 angstroms; thicker SAL elements are difficult to saturate for acceptable separation distances between the MR element and the SAL, and for acceptable magnitudes of current flow in the MR element. In addition, it will be appreciated that the continuing effective operation of a magnetic head depends upon the stability of the magnetic parameters of the materials comprising the head. It is known that the field required to saturate a thin uniaxial film is proportional to the anisotropy field of the film. As mentioned above, CoX SAL elements exhibit instability of the anisotropy field when made thin enough to be suitable for SAL service. This instability is reflected in changes in the saturation of the layer with resultant instability in the bias of the magnetoresistive element leading to undesirable variations in the head response.
The present invention addresses this problem, and provides a solution by disclosing optimally dimensioned CoX alloy films for use as SAL elements, so processed to have anisotropy fields that are stable and substantially immune from long term changes.